Among systems in which power is wirelessly transmitted from a power transmission device to a power reception device, there is a system that uses an electric field coupling scheme. In this system, electrodes of the power transmission device and electrodes of the power reception device are electric field coupled with each other and power is transmitted therebetween. In addition, in this electric field coupling scheme, if the amount of power being transmitted is comparatively large and a foreign object is interposed between the electrodes, a malfunction may occur. Accordingly, a wireless power transmission system capable of detecting that a foreign object has come close to an electrode is proposed in International Publication No. 2012/070479.
In the wireless power transmission system described therein, a foreign object detection electrode is provided in the power transmission device so as to be spaced apart from an electrode used for electric field coupling. A voltage generated between the electric field coupling electrode and the foreign object detection electrode is monitored and when this voltage changes by at least a certain amount, it is determined that a foreign object has approached the space between the electric field coupling electrodes of the power transmission device and the power reception device and power transmission is stopped.
However, there is a problem in that a separate foreign object detection electrode needs to be provided and needs to surround an electric field coupling electrode and furthermore the shapes of the electrodes used in the power transmission device are restricted. In addition, if the foreign object detection electrode has a cut-out portion, it will not be possible to detect a foreign object when a foreign object approaches the cut-out portion. Consequently, it may not be possible to detect a foreign object with certainty and there is a risk of malfunction.
Accordingly, an object of the present invention is to provide a power transmission device and a wireless power transmission system capable of preventing a malfunction when a foreign object has become interposed between electrodes with a high degree of certainty.